1. Field of the Invention
The invention relates to a levitation process and device for producing fire-polished gobs from low-viscosity glass, which are used in particular as preforms for the production of precise-pressed optical components with high-quality optical properties.
2. Description of Related Art
It is known to precise-press preshaped glass slugs (referred to below as gobs) to produce optical components with high-quality optical properties and a highly accurate surface structure and quality, for example for the production of refractive and diffractive components and lenses, in particular aspherical lenses.
Precise-pressing obviates the need for time-consumingand expensive milling, grinding and polishing for precise formation of at least one surface of the optical component. Precise-pressing is generally effected by reheating the gob and pressing it into high-quality molds. This procedure is used in particular if the milling, grinding and polishing to form relatively complicated surface structures of the component would be difficult and laborious.
High demands are imposed on the homogeneity and surface quality of the gob, since defects in the gob are transferred to the end product and lead, for example, to imaging distortions. This would mean that the end product would no longer be usable.
The gobs are usually obtained direct from the glass melt, by cooling a glass slug which has been fed into a mold without direct contact with a mold surface, giving the gobs what is known as a fire-polished surface.
In this context, production is effected using the levitation process, in which a gas cushion is built up between the glass slug which has been fed in and the mold, so that direct contact is avoided. The mold is designed as what is known as a levitation mold, i.e. is permeable to gas, so that a suitable volumetric flow of gas can emerge at the mold surface. The molten glass, which has been fed in portions into the levitation mold, cools down in the levitation mold and, under the surface tension forces of the glass, the force of gravity and pressure of the gas cushion which forms between the mold surface and the glass slug, adopts approximately the shape of the mold, while the gas cushion prevents it from touching the mold.
By way of example, to feed the glass slug into the levitation mold, it is known for a glass slug which has been portioned by means of a needle feeder to be caught in the levitation mold through which gas is flowing. During the filling operation, the levitation mold is moved downward, in such a manner that the distance between the top side of the glass slug and the feeder nozzle remains constant. When the needle closes, the glass slug at the end of the feeder contracts and becomes detached. To assist with this detachment, the levitation mold can be moved downward at an increased velocity.
It has long been known to use the levitation process to produce fire-polished gobs and to process them further by precise-pressing. A corresponding process and molds which are suitable for carrying out the process are described, for example, in DE-C 24 10 923.
However, the procedure has proven extraordinarily complicated, in particular for low-viscosity glasses, on account of the complex properties of the glass, in particular during the infeed and cooling process.
The procedure involved in producing the gob is substantially dependent on the quantity of the glass slug, the shape of the gob to be produced and the viscosity properties of the glass. Problems arise in the procedure both when the glass slug is being fed into the mold, during optimum production and setting of the gas cushion required and also with regard to the control of the profile of the cooling and shaping process. An imbalanced procedure leads to gobs of reduced quality, with bubbles, cords, surface defects and/or cooling waves.
To improve the procedure when feeding in the glass and shaping the gob, it is proposed in US 2002/0062660 A1 that the flow rate of the gas be controlled in a targeted manner to produce an optimum gas cushion. In a first step, a molten glass slug flowing downward is caught in a levitation mold, in a second step the mold is moved downward at an increased velocity (higher than the flow rate of the glass slug flowing downward), and in a third step the glass slug which has been fed in is shaped on a gas cushion. The first step is carried out at a lower gas flow rate than the flow rate in the third step; the flow rate in the first step may even be zero.
However, with a procedure of this type, the glass slug can come into contact with the mold while it is being fed in, which leads to a reduced quality, i.e. to cooling waves and surface defects in the gobs. Moreover, during a procedure of this type glass can stick to the mold, locking the openings in the mold. Consequently, the molds can no longer be used or at least have to be laboriously cleaned.